900 MHz Miniaturized Rectenna Abderrahim Okba, Alexandru Takacs, Hervé Aubert To cite this version: Abderrahim Okba, Alexandru Takacs, Hervé Aubert. 900 MHz Miniaturized Rectenna. IEEE Wireless Power Transfer Conference (WPTC), Jun 2018, Montréal, Canada. 10.1109/WPT.2018.8639385. hal-02066056 HAL Id: hal-02066056 https://hal.laas.fr/hal-02066056 Submitted on 13 Mar 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 900 MHz Miniaturized Rectenna A. Okba1, A. Takacs1, H. Aubert1 1LAAS-CNRS, Université de Toulouse, CNRS, INPT, UPS, Toulouse, France Email: [email protected]; [email protected]; [email protected] Abstract—This paper addresses the design and the characteri- same physical length, the lower operating frequency of the zation of a new topology of compact rectenna used for electro- antenna is smaller than one of the standard dipole and the magnetic energy harvesting of low incident electromagnetic pow- er densities. The rectenna uses a broadband miniaturized flat antenna bandwidth is increased. In this work, the designed dipole antenna with a single diode rectifier. The experimental antenna covers the 868MHz-915MHz ISM frequency band. It results demonstrate that the efficiency of the proposed compact was rigorously simulated by using the commercial software rectenna is up to 38% at 900MHz for electromagnetic power HFSS [3]. density of 0.26µW/cm². Index Terms—energy harvesting, rectenna, wireless power transmission, flat dipole antenna. I. INTRODUCTION Recently, Wireless Power Transfer (WPT) and electromag- Fig. 1. Sketch of two paths of the current density on the radiating surface netic Energy Harvesting (EH) have become an attractive solu- of the Flat Dipole Antenna tion for many industrial applications. The 3D indoor localiza- B. Miniaturization of the Flat Dipole Antrenna tion is one of these key applications. Nowadays, the 3D locali- zation systems use batteryless tags and beacons in order to The miniaturization of antennas can be performed from the derive the position of tagged objects in a warehouse. Bat- use of dielectrics [4], magnetic materials [4], or metamaterials teryless tags collect the power from the surrounding electro- [5]. The modification of the antenna geometry may also be magnetic field generated by radiofrequency (RF) dedicated applied (see, e.g., by loading the radiating surface by slit [6], sources. Once enough power is harvested, tags wirelessly by designing highly irregular antenna profiles [7] or by using transmit their positions to beacons. The beacons communicate coupled ring resonators [8]). The miniaturization of the Flat with each other through RF signals allowing the system to Dipole Antenna (FDA) is performed here by adding a metallic locate the objects. The RF sources generate the ambient elec- rectangular ring around the antenna, as shown in Fig. 2. This tric field in the 868MHz - 915MHz ISM frequency band. A allows reducing the physical length of the antenna of 25% compact rectenna composed of a Flat Dipole Antenna sur- while keeping the gain unchanged. Indeed, from appropriate rounded by a rectangular metallic ring and a rectifier with a design, the ring may favorably participate to the radiating field single diode is used here to harvest the ambient electromag- by increasing the antenna gain. netic energy in order to supply the batteryless tags. In this paper, the broadband Flat Dipole Antenna is present- ed in section II while the rectifier is studied in section III. The compact rectenna is detailed in the section IV and obtained experimental results are finally discussed. II. ANTENNA DESIGN AND RESULTS Fig. 2. Layout of the FDAs without rectangular metallic ring (purple) and with the rectangular ring (turquoise blue). The two antennas share the same A. Flat Dipole (FD) Antenna lower frequency and gain. Recently, broadband dipole antenna topologies have been The antenna is fabricated by using the lossy FR4 substrate reported for electromagnetic energy harvesting [1]–[2]. The (substrate thickness: 0.8mm, relative permittivity: 4.4 and loss Flat Dipole shape is carried out by changing the geometry of a tangent: 0.02). The size is of 10.5 x 6 cm². Fig. 3 displays the standard printed half-wavelength dipole. It is obtained by return loss with and without the rectangular ring. As expected, giving a round shape to the two constitutive quarter- the ring allows increasing the antenna bandwidth and reducing wavelength monopoles, as illustrated in Fig. 1. Contrary to the physical length of the antenna. The impedance matching is the standard dipole where the current density flows along the achieved between 840 MHz and 1.2 GHz. The radiation pat- dipole axis, the current in the shaped dipole flows by follow- tern is similar to one of the standard half-wavelength dipole ing different paths. As sketched in Fig. 1, the current density with a maximum simulated gain of 2.8 dBi at 900 MHz (see Js2 goes through a longer path than Js1. As a result, for the Fig. 4). 0 The RF-to-DC conversion efficiency can be derived from -5 the following expression: -10 -15 (%) = 100 (1) 퐷퐷 S11 (dB) -20 푃 where P is the measured휂 DC power∙ and P denotes the -25 푃푅푅 RF power injected at the rectifier input port. The measured -30 DC RF 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4 1,5 and simulated efficiencies are reported in Fig. 7. The obtained Frequency (GHz) maximum efficiency is of 38.6 % at 880 MHz. Fig. 3. Simulated return loss as a function of the frequency for the FDA without rectangular ring (red plot) and with rectangular ring (blue plot) 16 14 12 3 10 2,5 8 6 2 4 1,5 DC power (µW) 2 1 0 Gain (dBi) Gain 0,8 0,85 0,9 0,95 1 0,5 Frequency (GHz) 0 0,8 0,85 0,9 0,95 1 Fig. 6. Measured (blue plot) and simulated (red plot) harvested DC pow- Frequency (GHz) ers as a function of the frequency. Fig. 4. Simulated maximum gain of the FDA with the rectangular ring as a function of the frequency for θ=0° and φ=0°. 50 40 30 III. RECTIFIER DESIGN AND RESULTS 20 The rectifier was simulated using the commercial software 10 ADS. It is composed by the HSMS2850 Schottky diode (%) Efficiency 0 mounted in series configuration, a low-pass filter (100pF ca- 0,8 0,85 0,9 0,95 1 pacitor) used for the filtering of the fundamental and the har- Frequency (GHz) monics and, an adjustable resistive load (0 - 10kΩ potentiome- ter). The impedance matching circuit is composed of a short- Fig. 7. Measured (blue plot) and simulated (red plot) efficiencies of the circuited stub bent, and a 30 nH inductance for matching the rectifier as a function of the frequency input impedance of the rectifier at 900 MHz. Fig. 5 displays the simulated return loss of the rectifier. A good input match- IV. RECTENNA: FABRICATION AND MEASUREMENT RESULTS ing is obtained between 860 MHz and 910 MHz. As a first step, the rectifier and the tapered transition for 0 feeding the antenna are assembled and fabricated on the same substrate (Duroïd 5870). The resulting 3D antenna is shown in -10 Fig. 8. Its size is about 10.5 x 6 x 7 cm . -20 3 -30 S11 (dB) -40 -50 0,8 0,85 0,9 0,95 1 Frequency (GHz) Fig. 5. Simulated return loss of the rectifier as a function of the frequency The measured DC power is reported in Fig. 6 as a function of the frequency. The DC power of 12 µW is measured at 880 MHz with the 5 kΩ load (optimal load) and the input RF pow- Fig. 8. The fabricated rectennas: (a) the 3D view of the rectenna (b) top er of -15 dBm. An acceptable agreement is observed between view of the rectifier before mounting the lumped components and (c) bottom simulation and measurement results. view of the rectifier The experimental setup is shown in Fig. 9. The Anritsu 35 MG3694B microwave generator is used for injecting the RF 30 signal at the input of the transmitting (Tx) horn antenna (1–12 25 GHz) via a coaxial cable. The horn antenna illuminates the 20 rectenna under test with a linearly-polarized E-field. An auto- 15 matic acquisition routine is implemented in Labview software 10 DC power (µW) 5 from National Instruments to speed-up the acquisition process. 0 The harvested DC voltage is measured by using a standard DC 0,8 0,85 0,9 0,95 1 multimeter. The DC power can be derived from the measured Frequency (GHz) DC voltage as long as the load impedance is known. The Fig. 10. Measured DC power at the input port of the resistive load measured insertion losses due to the coaxial cable are of 1dB (RL=300Ω) as a function of the frequency in the entire frequency band of interest. The optimal load impedance was also experimentally de- termined. Fig. 11 shows the DC harvested power as a func- Tx antenna tion of the resistive load value at 900 MHz. It can be observed Rectenna that the maximum DC power is obtained for R=5 kΩ. RF power generator 35 30 25 20 DC multimeter 15 10 DC power (µW) 5 0 1000 3000 5000 7000 9000 Fig.